Hydrogen is utilized in a wide variety of industries ranging from aerospace to food production to oil and gas production and refining. Hydrogen is used in these industries as a propellant, an atmosphere, a carrier gas, a diluents gas, a fuel component for combustion reactions, a fuel for fuel cells, as well as a reducing agent in numerous chemical reactions and processes. In addition, hydrogen is being considered as an alternative fuel for power generation because it is renewable, abundant, efficient, and unlike other alternatives, produces zero emissions. While there is wide-spread consumption of hydrogen and great potential for even more, a disadvantage which inhibits further increases in hydrogen consumption is the absence of a hydrogen infrastructure to provide widespread generation, storage and distribution.
One way to overcome this difficulty is through the operation of hydrogen energy stations. At hydrogen energy stations, reformers are used to convert hydrocarbons to a hydrogen rich gas stream. The hydrogen rich gas stream can be purified to a high purity product. The gaseous hydrogen is then stored in stationary storage vessels at the hydrogen energy stations to provide inventory to fuel hydrogen vehicles. The stationary storage vessels may be constructed of steel or a composite material. Station operators must be able to calculate the amount of gaseous hydrogen stored at the hydrogen energy stations. In addition to other considerations, an accurate hydrogen storage calculation is necessary for leak checking on the storage vessels.
The inventory of gaseous hydrogen in a stationary storage vessel is commonly determined using the external ambient temperature and the internal pressure to calculate the mass of hydrogen in the stationary storage vessel. To make this calculation, the ideal gas law, PV=nRT, may be used. However, the ideal gas law is not accurate at high pressures. As a result, the inventory of gaseous hydrogen in a stationary storage vessel is commonly determined using an equation of state. One example of an equation of state is the modified Benedict Rubin & Web equation of state:
  p  =            ρ      ⁢                          ⁢      RT        +                  ∑                  i          =          1                19            ⁢                        G          ⁡                      (            i            )                          ⁢                  ρ                      n            i                          ⁢                  T                      m            i                                +                  ∑                  i          =          20                32            ⁢                        G          ⁡                      (            i            )                          ⁢                  ρ                      n            i                          ⁢                  T                      m            i                          ⁢                  exp          ⁡                      [                          γρ              2                        ]                              (Standardized Equation for Hydrogen Gas Densities for Fuel Consumption Application by Eric W. Lemmon, Marcia L. Huber, and Daniel G. G. Friend of the National Institute of Standards and Technology.) However, this equation of state is inconvenient to use as it not only consists of 32 terms but also requires iterative calculations for the solution.
In the calculation, the external ambient temperature is used in lieu of the internal storage vessel temperature to eliminate the need to penetrate the high pressure storage vessel. In addition, the use of external ambient temperature also eliminates the need to install electrical equipment in the Class 1 Division 2 Group B area (as defined by OSHA regulations) around the storage vessels. While the use of external ambient temperature has advantages, it also results in an inaccurate calculation of the mass of hydrogen in the vessel due to the delay in heat transfer from the ambient temperature to the hydrogen gas within the vessel. This delay is due to the time it takes to transfer heat through the thousands of pounds of steel that make up the stationary hydrogen storage vessels to the kilograms of hydrogen gas inside the tank. Daily temperature swings of approximately 20° C. will affect the pressure in the hydrogen storage vessels. Due to the delay, peak pressure will lag peak ambient temperature as shown in FIG. 1 depicting data from a hydrogen energy station in Orlando, Fla. For example, for 300 kg hydrogen storage systems the variation in inventory can be greater than 2%.
The present invention addresses both of these concerns by providing methods for conveniently and accurately calculating hydrogen storage inventory in a stationary storage vessel.